Rainfall extremes linked to showers - Nature Geosciences

Rainfall extremes have far reaching consequences for nature and human society and their study therefore constitutes one of the main research focuses for meteorology and climatology. However, the topic of extreme rainfall is further complicated by rainfall properties being strongly dependent on the time scale studied, e.g. five minute periods compared to hourly or daily periods. There are also different types of precipitation, resulting from different processes that produce rain. Imagine a summertime thunderstorm (termed a convective type event) with sudden intense showers for a short while, and contrast that with rain from a wide-spread cloud cover (termed a stratiform event) with light on-and-off rainfall that persists for many hours. Both can produce extreme amounts of rain on the ground, but at vastly different time-scales.

The amount of rainfall from heavy showers is linked to the water vapour contained in the atmosphere. At higher temperatures, the atmosphere can hold more water vapour and – in principle - has the potential for producing heavier rainfall. It has previously, however, been shown (Lenderink and van Meijgaard, Nature Geosc., 2008) that the intensity of rainfall within a given hour is increasing faster than the increase in atmospheric water vapour with higher temperatures. It was argued that showery rainfall responds differently than large-scale events, which might follow the water vapour content closely. No observational evidence was, however, presented and another study questioned whether the observed increase in heavy rainfall could not simply be due to statistical effects (Haerter and Berg, Nature Geosc., 2009).

In the current paper in Nature Geoscience (Berg et al., 2013), Peter Berg, Christopher Moseley and Jan O. Haerter investigated the behaviour of different types of rainfall events at varying temperatures. This was possible by using large data sets of rainfall measurements covering a large area, and weather observations to separate the rainfall types. They found that the showery convective rainfall type indeed intensifies much faster in response to warmer temperatures, especially for temperatures from roughly 12 to 20°C. Furthermore, when considering the rainfall over the complete duration of an event of persistent rain, the showery rainfall remains intense throughout the lifetime of the event, while the wide-spread events show no obvious structure. This means that showery events can potentially be more threatening because they will be extreme over extended times. The authors further show that spatial and temporal observations can be made compatible: Observing the statistics of rain at a given point yields similar insights as observing profiles of rain fields over large areas but at isolated times.

Although the study discusses the behaviour of rainfall extremes at different temperatures, the results are not straightforward to apply to a changing climate. For this reason, the authors encourage corresponding investigations using very high resolution cloud and climate models that can resolve the processes leading to the showery rainfall.